A gradual loss of genomic integrity has long been proposed to contribute to the aging process. Consistent with this theory, mutation of genes involved in maintaining genome integrity often leads to premature aging. Identifying the potential genetic predispositions to age-related diseases and, more importantly, understanding how these genetic determinants lead to disease will likely guide our ability to better predict, diagnose and slow the onset of these disorders. The ATR protein kinase maintains genome integrity in mammalian cells and is a central regulator of cell cycle checkpoints. Using a cre/lox system that allows conditional deletion of the ATR gene in 2-3 month old adult mice, our preliminary work has demonstrated that several age-related phenotypes appear 3-6 months after ATR deletion. These aging phenotypes include alopecia, hair graying, kyphosis, osteoporosis, cardiomyopathy, testicular atrophy and acute immune suppression. With this unique mouse model, we propose herein to substantiate these initial findings and further explore the role of ATR in preventing age-related diseases. Experiments are proposed to further explore the effect of ATR loss on several disorders that are components of human aging (hair loss and graying, osteoporosis and reduced hematopoietic regenerative capacity) and, importantly, to address if these phenotypes are caused by a loss of regenerative capacity following ATR deletion and/or a direct effect on differentiated cells. To determine if activation of the p53 transcription factor plays a causative role in age-related phenotypes that result from ATR deletion, the premature aging observed in ATR mice will be compared with that observed in mice that lack both ATR and p53. Finally, experiments are proposed that will determine whether the oxidative DNA base damage that results from normal metabolic processes is exacerbated by ATR loss, leading to highly toxic DNA double strand breaks. Such amplification of DNA damage may accelerate normal aging. To test this hypothesis, oxidative DNA damage will be reduced or increased by treatments that modulate intracellular hydroxyl radical concentrations in ATR knockout cells, and the effect of these treatments will be monitored by chromosome spread analysis. These studies will seek a molecular understanding of how ATR deletion leads to premature aging.